Stress-related psychiatric disorders, such as post-traumatic stress disorder (PTSD) and depression, are serious mental illnesses that occur twice as frequently in women compared to men. Despite this disparity, we do not understand the biological basis of these sex differences. Human studies have revealed differences in the levels of the neuropeptides orexins in anxious and depressed patients compared to controls. Additionally, animal studies have revealed that orexins contribute to the stress response and to anxiety-like behavior. We have obtained preliminary data indicating orexins are differentially expressed in the brains of male and female rats. Consistent with previous research, we found that female rats exhibit a higher hypothalamic pituitary adrenal axis response to repeated stress compared with males. Furthermore, females exhibited a stress-related impairment in cognitive flexibility but males did not. Therefore, we hypothesize that this sex difference in orexin expression contributes to the exacerbated stress response and poorer stress-related cognitive flexibility in female rats. The proposed research aims to elucidate how orexins contribute to sex differences in the stress response at both a single cell and behavioral level. Specifically, Aim 1 of this proposal will use in vitro electrophysiology to determine if sex differences are observed i the membrane and firing properties of orexin neurons before and after stress. Morphology of orexin neurons in males and females will also be examined using Neurolucida neuron tracing software. Aim 2 will directly test the hypothesis that increased orexin expression mediates the higher stress response and poorer stress-related cognitive flexibility. We will use DREADDS (Designer Receptor Exclusively Activated by Designer Drugs) technology to manipulate orexin action during stress. We expect that inhibition of orexin neurons in females will decrease the stress response and improve subsequent cognitive function. These experiments provide the first evidence linking orexins to sex differences in stress and cognition, but these results will also have immediate implications for treatment. Orexin antagonists are currently used to treat insomnia, so it is crucial we understand how these neuropeptides differ between males and females, as there may be a future need to develop sex-specific guidelines in dosing. Moreover, orexins may regulate stress responses, food intake, autonomic responses, and emotional memory and thereby contribute to a variety of psychiatric symptoms. In this way, targeting orexins may treat a broad range of psychiatric symptoms in a sex specific manner.